Well, this was a very strange situation for the Phantom Thieves to find themselves in.
They had all gathered together, as they usually did, to discuss important (and some not so important) matters, what to do next time they entered their current targets Palace, which request they should handle next time they were in Mementos, how much some of them hated studying (and how much one of them wanted them to take their studies seriously)
But their leader, Ren Amamiya, wasn't satisfied. Not because the meeting went poorly, it went very well in fact. No, Ren was dissatisfied because it was taking so long. And that just meant he'd have to wait even longer before him and the love of his life spent the night together.
They'd talked about it beforehand, with his lover making up some excuse to stay behind for a while longer after everyone else had left, then they'd enjoy each others "company" for the rest of the night, and then go out on a date the next day, which was a Sunday, so they'd have all the time in the world for each other.
The problem? Everyone was just lounging around talking! He'd let out some hints that they should probably all go home, like commenting on how "late it was getting", or asking "When do the subway's close again?"
But none of them seemed to pick up on it. Eventually, he said he had to go to the bathroom, while his love went outside and around the corner to "take a call".
After a brief back and forth on how they were possibly going to have the passionate night they'd been dreaming about with all of their friends, (who didn't know that they were dating) just hanging around the place. Ren heard movement going from his attic down and out of the building. After a brief investigation, he couldn't find any of his friends anywhere upstairs.
While his first instinct was to go looking for them, his "downstairs head" was doing most of his thinking at the moment, the prospect of laying with the love of his life that night overriding his concerns as to where his friends had gone.
He shot a message to his partner, telling them that the coast was clear. Unbeknownst to either of the teens, their friends were still there!
...Just reduced to only 3 inches tall!
Now the remaining Phantom Thieves had to survive at this size, all while their Leader had the time of his life with his partner...