The episode starts with Barry waking up from a nightmare of Iris dying at the hands of Savitar. This of course is a flash forward from Barry running into the future after throwing the philosophers stone into the "Speed Force" in the mid-season finale. The episode also covers the adventures of Wally West a.k.a. Kid Flash as he shadows Barry on how to use his powers. Wally and Barry are at odds with each other after Wally takes down plunder. In the future, the news station Barry was watching, one of the headlines said plunder was captured, so he believes if he lets plunder go he can change the future. HR opens the S.T.A.R. Labs museum but it attracts no visitors. The episode eventually leads to Barry telling Iris that he saw her die in the future and later, he lets the rest of the team know whats up. HR gives an example with dominoes on how the future can be changed, but it has to be done precisely in order to avoid the event. Cisco offers to "vibe" them to the future to write down all the events that lead up to Iris's death. Eventually Barry and Wally chase down plunder, but Barry insists that Wally capture him to change the future. Wally is able to take down plunder and put him in jail, which changes the future from "Flash takes down plunder" to "Kid Flash takes down plunder".
The mid season premiere of The Flash is messy and a bit weak. The feud between Barry and Wally comes out of nowhere and In my opinion is a bit convoluted and unreasonable on Barry's part. Plunder is taken down relatively easily and we only see him for a short amount of the episode. HR opening the S.T.A.R. labs museum and the eventual bonding between HR and the rest of the team is a bright spot. The team and HR have sort of been at odds with each other so its nice to see that the conflict is cleared up, and HR is cementing his place on the team. However, I feel HR is still hiding something and his venture to Earth-1 is more than just for writing a book. Barry attempting to change the future is similar to Barry changing the past by stopping his mothers murder at then end of season 2. This of course causes an alternate timeline, but Barry decides to revert it back to the original timeline. However after he did that there were tons of adverse effects. So Barry messing with the future will most likely also cause adverse effects. The mid season premiere of The Flash ultimately tries to add too many conflicts in to one episode.