^That's a very summarized version but there's a lot more to everything than that. I will say season 2 was not as tightly written as 1 and the narrative got spread thin partly due to cast size, but I still believe the series as a whole is worth watching.
Adding some extra context to Galeblue's response though, and heavy spoilers for the series here. Jinx kills Cait's mother at the end of season 1, and Vi at the start of season 2 says that she'll help Cait take down Jinx. For context Jinx is Vi's sister. So at the end of the day Vi can't take out Jinx due to still being family, and Cait filled with anger over her mother's death goes her own way leaving Vi behind. Vi becomes depressed and sinks her self into booze and pit fighting. Ambessa who is a general and leader from Noxus sees the weakness in Cait and uses that to whisper in Cait's ear, filling the hole in Cait with her mother being gone Ambessa use's Cait's weakness against her and steers Cait onto the path of a dictator. Various things happen, including Cait hooking up with another officer woman. Eventually Cait has a falling out with Ambessa and returns to her old self, correcting course, and the woman that Cait hooked up with was working for Ambessa so she's also gone from the picture. At the end of the day Cait and Vi get back together, they even have sex before the end fight and become a stable couple afterwards. The narrative is messy and not entirely the best handled, but I don't think it ruins Cait and Vi like some of the internet thinks. The characters are flawed and complicated, if the narrative was handled better and the plot wasn't so stretched thin with other plots such as the Jayce and Viktor one I think the series would have been very good all around. For me the series ends up being good, flawed, but again still worth a watch.