In this part, Krystal reveals that he was responsible for the always-sleeping Cub.
This part starts with Ginger sleeping in his bed, and David being awed at the sight. The Kaplan Boys then come to grips to the fact that they've been passive, by lounging and sleeping more than they were trying to help solve the Cub mystery. When David tells them that they'll be going to Romania, they decide to let Ginger sleep because he's been doing a lot of work. This gets them to invite another friend, who happens to be Soniclight.
The family then enters the guest room to get Soniclight. Jamie then gets freaked out about the likely fact that the room has more beds than 10 hotels or apartment buildings combined, suggesting that certain beds are several hours away from the entrance door. As David and Amanda comfort her, the family comes across a pentagonal door, out of which Soniclight appears. The boys get him out, and he happily looks forward to the adventure.
The family then starts leaving the house. Smalltail is about to cry when Amanda invites him to come along as well, because, among other reasons, his mother (Blanche Sun) is there. This makes him happy. The family then sets off. Meanwhile, Pop wakes up, starting to miss feeding Cub (because Cub always slept). Suddenly Lumpy and Disco Bear show up as cops (as in A Vicious Cycle), accusing Pop of getting Cub in jail at young age. This confuses Pop, but when they threaten to arrest him, he goes to the prison to bail Cub out, and get him back in his bed.
Meanwhile, the Kaplan family is on their way to the train station, while David reminds everyone the courtesy rule to let any sleeping people be. He also tells the boys that they can sleep in they wish to. When they board the train, the Kaplan Boys are wide awake, but Smalltail is sleeping with his teddy bear. Joshua and Jessica end up feeling lonely because of this. Meanwhile, Jordan K drinks so much lemonade that he ends up needing the bathroom badly. He panics in front of the door of the occupied train bathroom, which Risky later comes out of. Jordan K enters the bathroom and the scene ends there.
When the family arrives in Romania, Smalltail happily reunites with his mother (for one scene), and Haley (Layla's stepsister) is encountered for the first time. The Kaplan Boys see many parents of official tree friends in the nerve center, particularly Prune, Layla and Chortle in the green workroom. However, Jordan K spots that Julia herself is there. This causes the boys to go into a loving trance.
They start expressing their feelings of love through poetry, until they are interrupted by Risky. Risky tells the story of how he started out napping at a tree, but then he got hit in the head by acorns. So he left that tree and headed home, but then Winter invited him to her house, and he ended up having a very comfortable snooze in an angel bed. Julia starts telling a similar story in her case, where she wanted to sleep on a train to Manhattan, but then Jasmine interrupts her. She claims that she is supposed to be the love of the Kaplan boys. Julia and Jasmine then end up in a fight.
Later, David asks everyone in the nerve center to vote between Julia and Jasmine to be with the boys. Everyone votes Julia except for Harold, so then Julia wins. Albatross then ends up putting Jasmine in the very same jail cell that Cub landed in, (since Cub would have never disappeared if Jasmine didn't put him on the roof).
There is then a flashback to Happy Tree Town, where Ginger finally wakes up. He sees the note and despairs that he would like to be in Romania. He then prepares to write the Kaplan family back, while walking to Cub's room so see him back in bed. Meanwhile, Jordicc is holding a sign "The one and only human bean!" next to the Handy and The Mole's Goofy Ginger statue. Brainy is then seen cherishing his sons Toothy and Timid while they nap on the couch. He comes to grips that family naps are not of major importance, and then gains total regret from the time he threw Cub out of the bedroom window. Afraid he got the baby into a coma, he tears up and runs outside to investigate.
The scene then returns to Romania, where several characters are seen sleeping and a few awake in Romania's Overrated Pavillion. Julia and the Kaplan Boys are chatting; as are Prune, Layla and Chortle. Also, Sniffles is telling Risky that his father (Krystal) is behaving suspiciously and would take a deep breath every time he wanted to say something.
Meanwhile, David gets off the phone with Pop and is apparently told that Cub is back in his bed at home, so that Dr. Moon can finally use her wires. While they get ready to do this, however, Krystal stops them. With a sweat, he takes everyone to his scientific realm, where a nonsleeping Cub sits on the table with blocks. Krystal reveals that Cub was there all along, and that the always-sleeping cub who lived with the Kaplan family was really an advanced hologram that was programmed to sleep.
He tells the story that he was asked to babysit Cub in December, but when Cub napped, Krystal stopped paying attention to him, and then Cub ended up touching one of Krystal's electric gadgets and hurting his hand. Krystal didn't want to give Pop the impression that he was a mean babysitter, so he quickly programmed a hologram of Cub to fool Pop, but he programmed it to only sleep because he was short on time.
Upon hearing this story, the Kaplan Boys get mad at him for causing their family and Dr. Moon unnecessary trouble. This makes Krystal feel remorseful for what he did. Krystal returns the real Cub, and tries to come up with another way to make things up to the family. After thought he finally comes to a decision which is not revealed until after the credits.
The scene then cuts to the family being on the train back from Romania, where David explains that they'll be arriving late. Before the train starts, Ripley asks if it'll have snacks. When Hovee sees Cub sleeping in David's arms, he momentarily thinks David isn't holding the real Cub, even though he is. Meanwhile Risky and the boys chat about the fact that Ginger was unfortunately not on the trip, which is cut short when Jordan K points out that Smalltail is sleeping. The Kaplan Boys remark that they're still wide awake, while the camera shows that David is sleeping. Ripley then apologetically tells Snake-Tail that he doesn't think the train has snacks.
When everyone gets off the train that night, Cub and Smalltail stay sleeping, while Joshua and Jessica stay awake. David discusses that the family will put the babies to bed and then see whether Ginger is still up. Without thinking, David puts Cub in bed with the hologram, and then realizes that he unfortunately can't tell the Cub's apart, and neither can anyone else in the house. So Hovee just positively says that they'll know which Cub is real in the morning when Cub wakes up.
Meanwhile, Jordan K and Ginger are chatting about the trip. The scene then jump cuts to later, presumably after Jamie, Joshua and Jessica going to bed.
The Kaplan Boys are playing a video-game version of The Fate of April, specifically on the part where Danwersa and Aster are asleep. When Julia says she's going to bed, the boys wish her good night and continue playing the video game. David tells them that they earned it but that they shouldn't stay up too much.
After the credits, Krystal is seen selling numerous sleeping Cub holograms, clearly tagged with "H"s on them. The No Hands, No Eyes, No Brain trio, along with Russell and Brainy's family, happily buy some. Brainy notes that he finally feels much better.
- When the boys said "We should tag another friend along!" "And I know who it should be!", Smalltail smiled before they said "Soniclight!" because he thought they were talking about him.
- This part reveals that there are 7,128 beds in the guest room, and that:
- Coincidentally, the Lumpy and Disco Bear cops are involved with exactly the same characters they were in A Vicious Cycle (namely, Pop and Cub).
- Giggles, Zoe and Snowers are seen sleeping together in a cameo. (In Nemao's viewpoint, Zoe is Giggles' and Snowers' daughter.)
- Snicker is also seen sleeping. However, his parents are not with him.
- On the way to the train, Smalltail can be seen starting to fall asleep.
- This leads to Smalltail sleeping on the train. The scene where Joshua and Jessica feel lonely alludes to when Smalltail felt lonely around Cub in Part 1.
- Smalltail presumably put up a fuss about taking a bath while he was covered in popcorn butter, leading to him staying up late at night.
- Joshua and Jessica are wide awake because they slept all day the previous day (WHHBOLH Part 2).
- Elemental and Nuggy have a cameo (but they are awake).
- Risky completes the boys' poem with a rhyme:
- She's so beautiful... just the look in her eyes...
- Makes me think of... the sun in the skies...
- With her and us bonded, we three shall rejoice...
- Her heart will provide us with---- Hey, Kaplan Boys!
- The angel bed in which Risky slept was originally introduced in April Fool's Day 2. If the player reaches the dream room with good deeds, they will receive an angel bed; the player then sleeps in the bed to entail the good ending.
- Next to the cot in Krystal's scientific realm, there are Schlegel diagrams of three of the six regular polytopes in 4 dimensions: the 5-cell (simplex) made of tetrahedra, the 8-cell (tesseract) made of cubes, and the 16-cell made of tetrahedra.
- They are all visible in the flashback of Cub sleeping in the cot. But when Joshua and Jessica jump on it, the tesseract is blocked.
- When Krystal says "Have Cub back," Smalltail is clinging onto him, presumably because he feels grateful over knowing what's going on with Cub.
- Jordan K's "Let's stop talking, because Smalltail is sleeping now!" alludes to when he said "Let's stop talking, because Joshua and Jessica are still sleeping right now!" in Part 1. However, he is being serious this time.
- This is the only part of WHHBOLH in which Ginger and Smalltail sleep.
- Also, unlike the first two parts, none of Jordan K's characters (except David on the train) are seen asleep.
- There was originally going to be a cameo of these four sleeping together. The idea was dropped because of little interaction between Sleep Mast R and Lou-pandita.
- Skaggles was originally going to be seen on the train both ways.
- Prune and Mime were originally going to be seen sleeping together in the pavillion; this idea was dropped.
- One of the last scenes of the story was originally going to have Cub soil his diaper, then have the family disgusted yet simultaneously relieved that he didn't have a sleeping sickness, then have someone change his diaper.
- At the end, Brainy's dialogue is seen as "Let's by two of these, everyone!"
- If the spelling mistake were corrected, the two lines would be better aligned on both sides.
- Skaggles' nose isn't supposed to be a red heart (an error common to all of Sleep Mast R's content before March 2019).