‘Supergirl’ Season 2 episode 8 is not really the beginning of The CW ‘Invasion!’ crossover. The Flash begins with Mon-El (Chris Wood) and J'onn (David Harewood) tracking their big bad to Earth One, and carry a limp Supergirl through a breach seeking help from Team Flash. This week’s Supergirl episode was the beginning of the two-part crossover between Supergirl and The Flash.And I have to say that both the Supergirl episode alone and its companion piece, The Flash episode titled “Duet”, were pretty awesome. So I thought I not only recap Supergirl season 2 episode 16 but also briefly review The Flash season 3 episode 17. ‘The Flash’ Season 3 episode 8 is where it all starts.