I applied online. The process took 2-3 weeks since I submitted my resume until I got an email with the sentence "After reviewing your work and experience, we've made the decision to not move forward at this time."
The process to coordinate interviews was very efficient. I had four phone interviews: one with a recruiter and three with technical people. They were nice during the interviews.
The interviews were easy: I had to explain my previous experience and answer technical questions about C++/STL, finite element method, and computational geometry. You do not need a specific training or preparation for the interviews.
I asked each interviewer about the pros and cons of the company. They did not mention any significant cons. They enumerated a lot of pros: benefits, impact of the work, work environment, etc.
Question to avoid (the interviewer was kind of embarrassed when answering the question). Some context: The main reason of Plethora's existence is the automation of CNC machining (the company is working hard on this, but still does not know when will be able to do it). I asked one of the interviewers about the difference with Protolabs, a competitor that has already automatized CNC machining. In addition, Protolabs also has an automatized service for on-demand manufacturing of parts using 3D-printing and injection molding. The interviewer told me Protolabs uses a pedestrian method that works but does not allow for very small tolerances. On the opposite, the method Plethora is developing will be more elegant and efficient, and will also allow for a better part finishing.
Personal opinion: If you have a MSc/PhD in Computer Science/Mathematics/Engineering, probably you are overqualified for the job.