Securely store passwords for remote use

I'm building a nodejs service with a web control panel. The nodejs server needs the user's username and password to log them into a remote service. So I need their usernames and passwords in plain text. I have no control over the remote service, they only accept plain text data.

How could I store their passwords and usernames in a way the application can decrypt them, but in a secure way.

I might need these details while the user isn't on my site, because the nodejs script might need to relog.