I've been watching clips of the show lately, mainly dealing with Optimus Prime's plot line. Cartoon Network is only up to episode 19 I believe, so there will be spoilers up through episode 26 here...
Earlier in the season we see Optimus is training with Micronus Prime in the Realm of the Primes. For what task though, Micronus refuses to tell him. Later on, Megatronus contacts Steeljaw, telling him he caused the Alchemor to crash on Earth in order to free the Decepticons so that they could free him from his prison in another dimension. Steeljaw isn't really interested in helping Megatronus, but agrees to do so if he's allowed to keep the Earth to which Megatronus agrees. Eventually Optimus is summoned before 6 of the 13 original Primes to undertake his mission, but they feel he still isn't prepared. Micronus explains they need to share a portion of their spark's with Optimus, but if he isn't ready, the power will kill him (even though he's basically dead already). Optimus, ever ready to sacrifice himself to save others, tells them to do it anyway and is upgraded to a new form resembling his bulked up form from Prime and is sent to Earth.
Meanwhile, the Decepticons free Megatronus who reveals his plot is to draw out the remnants of Unicron's spark from the Earth's core (even though Predacon's Rising established Unicron's spark is now contained in the former container for the Allspark after he'd possessed Megatron's body) and summon the Allspark from Cybertron to fuse them together (apparently Megatronus wants revenge against both Unicron and Primus for his 'humiliating banishment', and says the Earth played a role in that, even though Prime established the Earth was formed around Unicron's body after he'd been defeated by the 13 Primes and was left floating in space). Steeljaw realizes this will destroy both planets and turns against Megatronus. Optimus is able to destroy Megatronus' spark fuser at the last moment, restoring the Allspark to Cybertron and Unicron's spark to the Earth's core. Enraged, Megatronus goes all out against the Autobots, but the power of team work defeats him. After the Autobot's hit him with a powerful blast, only a crater remains...
With the crisis over, the Autobot's talk about returning home, but there are still Decepticons at large on Earth so they all volunteer to stay. Bumblebee tells Optimus they need him and he agrees to join Bumblebee's team, but only as Bumblebee's equal, not as their leader.
While I haven't seen all the episodes in the season, I have to say I'm disappointed with it. I thought that it started out with a lot of potential, but I don't feel they utilized it to the fullest. It almost felt like another season of Prime, only instead of the race to get the MacGuffin of the week, it's the new Decepticon of the week. Although I will say it's nice to see so many new characters getting introduced, unlike Prime that hardly ever expanded its cast. It also seems almost odd Optimus would have to go through special training from several of the original Primes to deal with this threat when he already had to deal with a lot worst in the previous series with out any help from them. I was hoping if they were going to use the original Primes that we'd get to know a little more about them, or at least see them each training with Optimus as the season progressed. But he's hardly in any episodes as it is and the only other Prime we see, outside of shadow, is Micronus.