Infiltration of Phantom Lord is an event that takes place during the Phantom Lord arc.
After the abduction and rescue of Lucy and the defeat against Phantom Lord, everyone in the guild seems hopeless in winning the war. They are all then shocked when they see the Phantom Lord Guild walking towards the Fairy Tail building, not knowing what to do. Jose Porla, Phantom Lord's guild master, then orders the firing of Jupiter against Fairy Tail. Erza tells everyone to get down and sacrifices herself, using her body and her Adamantine Armor to protect their guild. Seeing this, Jose tells the Fairy Tail Mages that they don't stand a chance, and demands them to hand Lucy over, to which they all disagree, saying that they would rather die than to give their comrade, making Lucy cry. Jose, angry about this, then threatens them that they will fire Jupiter again in 15 minutes, and releases his Shade Troopers to attack the Fairy Tail Mages. Natsu, Happy, Gray and Elfman then set out to infiltrate the Phantom Lord Headquarters and destroy Jupiter while the rest of the Guild stays to fight the enemy.
With 14 minutes until the launch of Jupiter, Natsu stands on top of the barrel of the cannon, punching it continuously but with no effect. Happy suggests breaking it from the inside, leading the two of them to infiltrate the enemy guild's building through the muzzle. As they continue down the metallic path, they arrive at a room with large spherical pieces of Lacrima which Happy deduces to be used for the firing of Jupiter. Natsu says that all he has to do is break them but he is stopped by Totomaru and they both then proceed to battle During the confrontation, Natsu learns to null Totomaru's control over his flames and fires a large beam of fire at the Lacrimas, destroying it and successfully stopping Jupiter.
The Phantom Lord Headquarters proceeds to transform into Super Mage Giant Phantom MK II, which incapacitates Natsu as he can't deal with mobility and allows Totomaru to take advantage of that. However, Elfman and Gray arrive and finish off the Fire Mage. The Super Mage Giant Phantom MK II starts casting Abyss Break, a forbidden Magical spell, leading the four infiltrators to split up inside Phantom Lord to find the guild's power source and destroy it. Elfman is found by Element 4 member Sol of the Earth, who specializes in Earth Magic, but Elfman defeats him after reawakening his Full-Body Take Over. Mirajane notices Sol's defeat slows down the progression of Abyss Break, prompting her to conclude that by defeating all of the Element 4 the forbidden spell will be stopped. Meanwhile, Natsu continues through Phantom Lord until he meets Aria of the Heavens, pinnacle of the Element 4.
At the same time, Gray encounters Juvia of the Deep, Water Mage of the Element 4. The two Mages battle but Gray comes out victorious. Natsu fights Aria, the last standing Element 4 member, but is unable to defeat him. However, Erza arrives and finishes him off.