Robotech II: The Sentinels was an attempt by Harmony Gold USA to continue the original 1985 Robotech television series. Only three episodes were ultimately animated before the project was canceled in 1986, and a feature-length film was released from footage taken from the completed episodes. The aborted 65-episode Sentinels series would have followed the ongoing adventures of Rick Hunter and Lisa Hayes and the rest of the Robotech Expeditionary Force (REF) during the events of Robotech: The Masters and Robotech: The New Generation series. Much of the series' failed production was detailed in the reference book, Robotech Art 3.
The "Pioneer Expedition" was a mission to confront alien forces in space to prevent interplanetary war on Earth. It took place concurrently with the Second and Third Robotech wars. The expedition was led by Lisa Hayes aboard the Super Dimension Fortress-3 Pioneer (SDF-3), which was launched in 2022 in search of the Robotech Masters' homeworld. The mission soon became preoccupied with the liberation of worlds under the control of the Invid Regent. The final battle of the Second Robotech War caused a release of the spores of the Invid Flower of Life across Earth, which soon attracted the Invid. In 2031, under the control of their Supreme Ruler, the Regess, they invaded Earth. The United Earth Forces (including the elite Army of the Southern Cross) had been severely weakened by the Second Robotech War and were only able to offer limited resistance. The Invid seized control of the planet and began utilizing the human population for slave labour and scientific experiments, resistance fighters tried to retake the planet with little success. The Robotech Expeditionary Force (REF) sent Mars Division to reinvade the planet, but their operation ended in disaster. It was only with a significantly larger REF force, now equiped with Shadow Devices and Synchro Cannons, who was willing to deploy Neutron-S Missiles, weapons of massive destruction, that the Invid were dislodged from Earth.
The feature-length pilot is composed of the first three (and only) episodes that were produced for the series. It introduces the SDF-3, along with its crew, and gives an overview of their new mission. The title refers to the Sentinels, an alien resistance movement encountered by the Robotech Expeditionary Force (REF) that consists of races subjugated by the Robotech Masters or the Invid. The significant events in the film include Lynn Minmei making peace with Admiral Lisa Hayes well enough to sing at her wedding to Major General Rick Hunter and the Invid's brutal invasion of the Robotech Masters homeworld of Tirol.
Being a sequel/spinoff to the combined series, The Sentinels featured characters from all three Robotech sagas, including Rick Hunter, Lisa Hayes (later Lisa Hayes-Hunter), Max Sterling, Miriya Parina Sterling, Exedore and Breetai from The Macross Saga, Dana Sterling, Bowie Grant, and a couple of their superiors from Masters, in addition to Jonathan Wolff and the Invid Regis of The New Generation (Scott Bernard was also planned, though never appeared in the confines of the feature). Among the newly created characters were young cadet rivals Jack Baker and Karen Penn, whose early love-hate relationship mirrored Rick and Lisa's; Vince Grant, brother of Claudia Grant, and father of Bowie Grant; and the Invid Regent, the villainous leader of the Invid. Dr. Emil Lang, a supporting character in the Macross Saga, would return as a main character. The story also introduces a human adversary in the form of T. R. Edwards, who was first introduced in Comico's Robotech: The Graphic Novel.
The Tatsunoko Production animation studio assigned the first script drafts to writers Sukehiro Tomita (Macross, Mospeada) and Hiroshi Ohnogi (Macross). According to director Carl Macek, the Japanese animators initially tried to relate the project to the original versions of Macross, Southern Cross, and Mospeada, until Harmony Gold explained the differences made in Robotech's adaptation with diagrams and charts. When the animators focused on new characters instead of Rick Hunter and the other characters derived from the original Macross series, Macek ended up reassigning the scriptwriting to American writers, headed by script supervisor Kent Butterworth to refocus the project.
Upon viewing the completed animation, Macek felt that scenes featuring new characters (such as Jack Baker, Karen Penn and The Invid Regent) had received much more care and attention by the animators than those with the original Macross characters (Rick Hunter, Lisa Hunter, Max Sterling). Macek believed that Tatsunoko secretly intended to re-use this higher-quality footage for a Japan-only sequel to Genesis Climber MOSPEADA - one of the Japanese series that had been adapted into the original Robotech series.
Macek blamed the cancellation of the series on the crash of the Dollar/Yen exchange rate in the mid-1980s, which caused toy partner Matchbox to withdraw from the project due to the increased cost. Since Harmony Gold lacked the funds to produce the series on its own, production ceased after only three episodes.