In 1 Peter 3:18-22 we come across one of the most disputed passages in the entire New Testament. It starts off just fine — it looks just like chapter two’s connection of the suffering of household slaves with the suffering of Jesus. And here too: as you are suffering, so also Jesus. And then the similarities shift. Overall, though, the big idea is this: we are to be emboldened because Jesus has gone before us.
Christ suffered for sins; the just for the unjust; and he suffered in order to lead “you” to God. Leading from one thought to yet another: suffering of Christians, suffering of Jesus, Jesus’ saving work, spirits in prison, et al.

Let me suggest the big picture of what is going on: the best term to use is recapitulation or incorporation. In other words, Peter has here a kind of Second Adam Christology going on.
Christ suffered; he was just; his suffering is for sins and is designed to take us with him to God. He was put to death in the flesh; he was made alive in the Spirit. Thus, death and resurrection/ascension. In this death to ascension of Jesus the Christians participate if they are in Christ. Hence, the Christians can endure suffering with hope because they know the way of Jesus.
Along with this big point Peter adds a comment that sets off a comment and digression: Jesus was made alive in the Spirit, and in this Spirit Jesus also preached to the spirits in prison. And I think the verbs/participles of motion indicate that he did this after his resurrection and prior to sitting down at the right hand of God (3:22).
Peter defines the spirits — and here he sounds like 1 Enoch — as those who disobeyed during the days of Noah, and he suggests they are in a “holding cell” until Jesus announces his victory and liberates them from their imprisonment. And while he’s talking about it, he brings up an analogy: just as Noah and his family was saved through water, so Christians are saved by water now (baptism). And while he’s talking about baptism, he defines it as not simply the removal of dirt from the body but as a pledge to God.
He then reverts back to resurrection (dropped after 3:18) and proceeds to the ascension and has Jesus sitting in victory (sounds like Daniel 7 and 1 Cor 15 and Phil 2:11).
We feel a bit like the Pevensie kids who find themselves back in the wooden wardrobe, back in reality. Leading me to think Peter, all along was concerned with providing the grounds for the hope of the suffering Christians: Jesus has paved the way to God, and they can simply trust in his path as their path. Thus, this is Phil 2:6-11 in a different context.