Lab Rats
In the start (Season 1) Leo discovered Adam, Bree and Chase and the Lab Rats wanted to go to school and have a social start so season 1 had more of showing who they are. In a season 1 episode Marcus had been shown to be a bad guy to Leo but not to the Lab Rats but in more episodes they start doing more life-saving events and in the Season 1 finale they go to space. In Season 2 in "Bionic Showdown" Marcus becomes a threat to all of the Lab Rats and Donald as it starts with more villains like Marcus and Douglas. Then in the Season 2 final "No Going Back" the bad guys keep coming to try to get them and all season finales end with a cliff hanger with the series premiere of Season 3 (Sink Or Swim) continuing from the season finale of Season 2.