This is the second book in the Guardians of Terraria series. It picks up some time after the end of the first book. If you haven't read the first book, you might want to do so first. You can find it in my profile.
Below, I will include a summary of the first book largely to serve as a refresher. Or if you just want to start from here because Calamity, at least you'll get some context for what the characters have already been through.
So if you are planning on reading Guardians of Terraria I, look away now...
William, Robyn and Sid all wake up in the middle of a forest in the game of Terraria, with memories of the real world but no recollection of who they are in the real world beyond their names.
In order to stay alive, they play the game, building up a base, managing the NPCs (known in this series as citizens) that keep moving in, gearing up with weapons and armour, and fighting bosses, such as Skeletron, who had been contained inside a cursed Old Man Clothier. Inside the Dungeon, Robyn thinks she sees the Lunatic Cultist, but neither William nor Sid see him.
When fighting the Queen Bee, they rescue a fourth person – Meg – who had been trapped inside the hive. She goes on to join them on their adventures.
In the meantime, William and Robyn become close, Robyn starting to develop feelings for him.
The Guide eventually goes missing. The players track him to the Underworld, where he ends up falling into a pit of lava, summoning forth the Wall of Flesh. The fight is tough, but the worst is yet to come – after the fight, Robyn turns against them, eyes glowing red and murdering the Golfer in the process.
William, Meg and Sid quickly deduce she must have been cursed by the Lunatic Cultist, as the Clothier's eyes had been similarly red before they'd fought Skeletron to free him. What's more, the Dryad tells them the Wall of Flesh was the world's Guardian, and its defeat caused the world to drastically change, bringing forth much more powerful enemies.
William, Meg and Sid work to get even stronger. Robyn shows up a number of times to attack them – once during a Blood Moon, summoning creatures from the depths of the lake – and later she unleashes the three Mechanical Bosses upon them.
William, Meg and Sid try to search for the Lunatic Cultist, but are unable to find him, so they take a detour to the Lihzahrd Temple in the jungle. But all they find are bones of a massacred civilisation and a Golem damaged beyond repair (though still rather powerful).
Eventually, the Lunatic Cultist makes his grand appearance – walling in the Dungeon behind a defensive bastion. They manage to get past his army and reach the Dungeon entrance. William fights cursed Robyn while Meg and Sid take on the Lunatic Cultist. Meg and Sid are successful at defeating the Lunatic Cultist, which cures Robyn's cursing just seconds before she would have killed William.
However, the Lunar Pillars appear across the world. The four players bring them down, but right after Cthulhu – Lord of the Moon – arrives. The fight is brutal, but Robyn is able to stay fighting the longest and brings him down.
The players and the citizens celebrate, but there are still questions left unanswered...