Project Silent Sparrow is the name of a portal that was built by Krei Tech as part of Alistair Krei's attempt to create a new way of transportation on Akuma Island. Years later, Yokai rebuilt the portal as a weapon against Alistair.
The portal was constructed on the outskirts of San Fransokyo as a top-secret collaboration between Krei Tech and the government. There were two portals and as something went through one, it will out of the other. The scientists had planned to send a human pilot inside a pod through and out it. Even when anomalies were showing in the scans, Krei still went through with it to show the general that it was safe. But after the pod went in the scientists lost contact with it and the second portal destroyed itself, while the other one became a vortex and began sucking everything in until it was forcibly shut down, dooming the pilot in the space between the portals.
Blueprints and reports for the project were salvaged by Yokai during his time in hiding. Hiro Hamada saw the project's symbol prior to his first encounter with Yokai and later saw the masked villain bring one of the three parts of the portal to San Fransokyo Bay. After Hiro upgrading himself, his friends and Baymax, the team headed to Akuma Island and discovered the remains of the facility as they were trying to find Yokai. The team watched footage from the control pit and saw Alistair Krei demonstrating the portal to the garment.
The team's theory of Krei being the masked man grew as they "realized" the reason why Krei had started the fire that killed Tadashi Hamada and stolen Hiro's Microbots, so he could take his technology back, but Yokai turns out to be Professor Robert Callaghan and he escapes with the end of the portal when Baymax had his healthcare chip reasserted into him. After Hiro calms down and the team saw the rest of the footage, they realize why Callaghan wanted it: the pilot was his daughter, Abigail Callaghan and he blamed Krei for her "death" because he sent her into the portal despite knowing that it was unstable.
As Krei introduces his new Krei Tech Building, Callaghan uses the microbots to capture Krei, put together and to hold up the portal as it starts to suck the building into it. Hiro later gets the team to use the portal to their advantage as it sucks up the microbots; an idea that came to him when he was nearly sucked the portal himself. Callaghan is caught, but as the team try to find cover from the portal's growing destruction, Baymax picks up Abigail's life reading within the portal, meaning that she is still alive. Hiro and Baymax go into the portal to save her and discover that the portal contains a void universe. After the two find Abigail and are about to bring her home, Baymax's suit is damaged and uses his rocket fist (with his healthcare chip clutched in it) to send Hiro and Abigail back. The portal then explodes when the two get out safely through and Abigail is sent to the hospital. Meanwhile, a seemingly regretful Callaghan is arrested and taken away by the police.
- The portal's demonstration was set to open the film, with "Doctor" Robert Callaghan as its creator, but the directors replaced it with the Bot Fight scene. They thought that it would be better if the audience discovered both the portal and the incident with the Hiro and the team.
- After the pilot went into the portal in the original layout of the scene, "something" was coming out of it.
- The Silent Sparrow alludes to the sparrow of Japanese folklore, where the sparrow is said to be a symbol of unexpected fortune and a reward for kindness. In the context of the film, the use of a sparrow for the portal's symbol foreshadows the unexpected return of Abigail from her imprisonment. Because Baymax sacrificed himself for Abigail's life, the reward comes in the form of Hiro discovering Baymax's chip and using it to rebuild Baymax.
- In the Big Hero 6 manga Tadashi was secretly helping Callaghan with the portal so they could save Abigail. However the night when Hiro first discovers the portal, Tadashi was sucked into it. To make the portal work Yokai stole the city's power until Hiro and Baymax managed to fully it with a powerful bolt of lightning.
- When Hiro and Baymax went into the portal to save Abigail they kept their eyes out for Tadashi, but Baymax's scanner couldn't find him or any other sign of life in the dimension that they were in; which led Hiro to believe that his brother was in a different dimension.