The Invisible Robeast is the twenty-sixth episode of the lion series of Voltron: Defender of the Universe.
The witch Haggar is showing King Zarkon a seemingly harmless bird, much to his bewilderment. Until she uses her magic turn the bird invisible. Her plan is to plant a Robeast on planet Arus, Zarkon approves the plan. It lands on Arus with no one on the planet the wiser.
Allura is riding in a carriage with Lance, Pidge and Nanny. They are coming back from a trip to collect fruit. Pidge and some of the space mice are enjoying some of the fruit. The carriage passes by some children, who wave to Allura and the others. Allura waves back as the Robeast strikes. The Robeast spooks the horses by lifting the carriage into the air.
Pidge calls to the castle as the carriage gets tossed in the lake, stopping Pidge's call. Lance, Pidge and Allura help Nanny. Suddenly the Robeast grabs Allura, to everyone's shock. Pidge attempts to aim his blaster at the Robeast while Lance helps Nanny. Pidge shoots the Robeast making it visible before it drops Allura and backs off.
The Robeast then throws a boulder at the group. The group try to run, but Nanny trips and the others go back for her. Thankfully the Black Lion arrives in time to save them. Lance and Pidge get into the Yellow Lion and Allura and Nanny and the mice get into the Black Lion. They tell the others about the invisible Robeast and the Lions try to track it. Though the can find nothing on scanners or infrared. With that the group goes back to the castle.
Lotor arrives and transforms the Robeast, making it larger. He then sends it to terrorize the Arusian countryside.
The Voltron Force, Nanny and Coran look on from the control room. Keith says that this is a ploy by Lotor to lure Allura out, so the they agree to launch without her. The others launch their lions.
The Robeast has set a town on fire by the time the Voltron Force arrives. Thankfully the ground crew has evacuated the people. Keith and Lance fire blindly. Lance manages to hit it, the Robeast fights back. Keith orders a retreat from the town and they track its position by the footprints it leaves behind.
The team lures the Robeast to the desert. They wait Green and Yellow Lions. The sun casts the Robeast shadow and everyone attacks. The Lions pursue the fleeing Robeast. Suddenly the sky becomes clouded and the team loses the Robeasts' tracks vanish into the lake.
Pidge takes his lion into the water over Keith's warnings. The Green and Black Lions are grabbed by the Robeast. The Red and Yellow lions charge in to help but are captured as well. The Robeast smashes the Lions into each other, until Keith fires a missile at it and frees them all. The Robeast attacks the Black Lion, Keith then shoots at the Robeast. The other lions then unleash their firepower on the Robeast. Their weapons have no effect but the Robeast then flees to destroy the castle. The castle defenses attempt to hold off the Robeast, which is unaffected thanks to its invisibility. Allura calls the rest of the team back and they re-engage the Robeast.
Lotor orders the Robeast underwater, where it's strongest. It goes after the Blue Lion but the others lure it away. Allura decides to launch the Blue Lion. Lotor spots Allura and orders her captured. Allura is able to save everyone but is the attacked herself. Keith checks all the scan modes until he manages to uncover a weakness. Keith takes the shot and it hits. The Blue Lion is freed and goes hurtling down to the ground until the other Lions catch it.
The team then forms Voltron. Voltron fires the stingray missiles at the Robeast. Then they attack it with the lion torches. This sets the Robeast on fire, Voltron then forms the blazing sword. Voltron thrusts the sword straight through the Robeast's chest, destroying it. Coran commends Keith and the others for a job well done as Lotor escapes.
At a celebration at the front of the castle, Allura kisses Lance in appreciation for his efforts, Lance then talks about how great Voltron is.
"We better go and help Nanny, before she yells herself hoarse."
— Lance going to help Nanny
"This is for dunking us all in the lake!"
— Allura doesn't take kindly to being dunked into a lake
Coran: "I feel some royal reward is due those brave men of the Voltron Force."
Lance: "The real reward comes from knowing that the people of this planet are safe and secure once more, and that we all did our part to make it that way with the help of Voltron, Defender of the Universe!"
— Coran and Lance