have you played the first PMD games (pokemon mystery dungeon: blue/red rescue team)? if so, then you will know what it is. if you liked it, then you should get explorers of time. it's a great game.
if not, then you should ask your brother if you may play a bit to know what it is like, since the mystery dungeons are different that the "main" pokemon games.
first of all, you don't play as a trainer, but as a pokemon. and you go in dungeons where you have proceed through floors by finding the stairs. it's still turned based, so all wild pokemon aren't doing anything till you do something (thus using a item, attack, or take a step (yes a step, so you can think when you're in a though sittuation)).
however, you can't go with more than 4 pokemons (including yourself) in a dungeon, and you can have times where you have to fight against 10 or even more if you come in a monster house (places on a floor where suddenly a hella lot pokemons appears and attack you), so it can be a though game.
the game has also a great story, but I won't spoil anything for you. ;)
if you have any questions, then ask so.